Image converter electrode arrangement for a mass spectrometer



y 7, 1970 H. F. MAI ETAL 3,519,814

IMAGE CONVERTER ELECTRODE ARRANGEMENT FOR A MASS SFECTROMETER Filed Jan. 5, 1967 FIG. 1

INVENTORS HEEMANA/ F. MAI

HINZ K. WAG/V52 United States Patent 3,519,814 IMAGE CONVERTER ELECTRODE ARRANGE- MENT FOR A MASS SPECTROMETER Hermann F. Mai, Dresden, and Heinz K. Wagner, Jena,

Germany, assignors to Friedrich-Schiller-Universitat Jena, Jena, Germany Filed Jan. 3, 1967, Ser. No. 606,661 Int. Cl. HOlj 39/34 US. Cl. 250-413 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to mass spectrometer apparatus. More particularly, the invention relates to the image converter electrode arrangement of a mass spectrometer.

In mass spectrometer apparatus of known type including electrometric recording, an image converter electrode may be utilized to increase sensitivity. The image converter electrode is located outside the field of the ion analyzer and the secondary electrons are not subject to a magnetic guide field. Apparatus of this type permits the recording of a plurality of mass lines in chronological sequence only.

In a known mass spectrometer which utilizes photo graphic recording, the ion conversion effect may be utilized to increase the sensitivity. The combination of the image converter electrode with the ion analyzer provides an increase in sensitivity merely by utilizing secondary emission and the increased sensitivity to electrons of the photographic plate utilized for recording the spectra.

In a known mass spectrometer utilizing photographic recording, a specific segment of background blackening is influenced, in the vicinity of the lines of maximum intensity of the mass spectrum, by an electro static suppressor plate positioned between the photographic plate and the ion analyzer. This reduces, to some extent, background blackening in the vicinity of the lines of maximum intensity of the mass spectrum, but the remaining amount of background blackening remains as a significant mass of spectrum interference. The photographic plate may be provided with a conductive layer at the lines of greatest intensity in order to reduce background blackening, but this does not reduce background blackening for all elements. If a conductive layer is positioned between the supporting plate and the photo emulsion, does not reduce background blackening or blackened areas more than partially. The background blackening or blackened areas may be completely reduced by partitioning the photographic plate and mounting it in a modified plate holder to permit the ion beams of greatest intensity to penetrate freely to the wall of the magnetic analyzer. This entails difficulties and expense, since the partitioning of the photographic plate must be accomplished in complete darkness with a glaziers diamond.

The whole ion current may be indirectly measured by 3,519,814 Patented July 7, 1970 a slit diaphragm before it enters the mass separator and may be utilized as a relative measure for the number of singly charged atomic ions of the parent substance which are trapped by the photographic plate. The ion current of the singly ionized atoms of the parent substance may be electrometrically measured at the same time that a section of the mass spectrum is photographically recorded. As soon as the fractions of the individual beam components provided by the parent substance vary, the application of the whole ion current as a relative measure for the current of singly charged atomic ions causes errors in analysis. Electrometric measurement of the ion current of the singly charged ions of the parent substance after their escape from the magnetic field, with simultaneous exposure of the photographic plate, covers only the mass range of the spectrum which is below the atomic mass of the parent substance.

The principal object of the present invention is to provide a new and improved mass spectrometer apparatus. An object of the invention is to provide a new and improved image converter electrode arrangement of a mass spectrometer. The mass spectrometer apparatus of the present invention overcomes the disadvantages of the mass spectrometer apparatus of the prior art. The mass spectrometer apparatus of the present invention provides improved sensitivity and improved accuracy of measurement and eliminates background blackening in the vicinity of the mass lines of maximum intensity. The mass spectrometer apparatus of the present invention operates efliciently, elfectively and reliably, with great accuracy and sensitivity. The mass spectrometer apparatus of the present invention provides improved detection limits and considerably shortened analyzing time.

In accordance with the present invention, the image converter electrode means of mass spectrometer apparatus comprises an image converter electrode having a determined part positioned to deflect electrons in a determined direction. Ion source means directs a plurality of ion beams to impinge in a primary ion spectrum upon the determined part of the image converter electrode in a manner whereby the ion beams release secondary electrons upon striking the determined part of the image converter electrode and the secondary electrons are emitted in the determined direction. An electron-sensitive layer is spaced from the image converter electrode in operative proximity with the determined part thereof. A voltage source is connected between the image converter electrode and the electron-sensitive layer for providing an electric field in the determined direction for accelerating the secondary electrons. A magnet provides a magnetic field between the image converter electrode and the electron-sensitive layer for guiding the secondary electrons to the electron-sensitive layer to produce a scaled-down image of the primary ion spectrum thereof. A secondary electron emitter foil is positioned between and in spaced relation from the image converter electrode and the electron-sensitive layer and increases the secondary electron current released at the image converter electrode.

The image converter electrode includes a part positioned outside the beam path to prevent the most intense ion beams from impinging upon the image converter electrode thereby eliminating background blackening in the vicinity of mass lines of greatest intensity. The electronsensitive layer comprises photosensitive means for recording the secondary electro-m spectrum derived from the primary ion spectrum. The determined part of the image converter electrode is positioned at an angle of less than 45 with the focussing plane of the mass spectrometer apparatus. A window is formed through the determined part of the image converter electrode in the beam path of the ion beams in a position wherein selected ones of the ion beams pass through the window. Ion beams passing through the window are collected and are directly electrometrically measured.

In the method of mass spectrometer analysis of the present invention, secondary electrons are released in a secondary spectrum from a plurality of ion beams in a primary ion spectrum. The secondary electrons are accelerated in a determined direction and are guided in the determined direction. The secondary electron spectrum is recorded. Furthermore, the secondary electron current is increased. Determined ones of the ion beams are prevented from releasing secondary electrons. Determined others of the ion beams are prevented from releasing secondary electrons.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram, partly in cutaway per spective view and partly in section, of an embodiment of the image converter electrode arrangement of the present invention of mass spectrometer apparatus; and

FIG. 2 is a schematic sectional view of the embodiment of FIG. 1.

In FIGS. 1 and 2, a plurality of ion beams 1 are emitted by an ion source 11 (FIG. 2) and impinge upon an image converter electrode 3. The image converter electrode 3 is mounted between a pair of spaced pole pieces 2 of a magnet and is of substantially L-shaped cross-sectional configuration comprising a pair of planar members 3a and 3b of strip-like configuration joined along a longitudinally extending edge of each and extending at an angle of between 90 and 135 with each other.

The member 3a of the image converter electrode 3 has one or more slits, slots, windows or apertures 3 formed therethrough. The member 3a of the image converter electrode 3 is positioned at an angle a of less than 45 with the focussing plane 4. The image converter electrode 3 thus provides an increased electron current density by secondary emission.

The ion beams 1 release secondary electrons upon striking the surface of the member 3a of the image converter electrode 3. The secondary electrons released at the image converter electrode 3 are accelerated by an applied electric field ve (FIG. 2) toward an electron-sensitive layer 5 of a base plate 6. The electric field ve may be provided by any suitable means such as, for example, a source of voltage 12 electrically connected between the image converter electrode 3 and the electron-sensitive layer 5. The magnetic field provided by the pole pieces 2 of the magnet closely guides the secondary electrons in the general direction of the lines of force of said magnetic field.

The electron-sensitive layer 5 may comprise one or more photographic plates or films and is utilized to record the primary ion spectrum converted to a secondary electron spectrum. The utilization of secondary electrons permits the greater measuring accuracy attainable with electron-sensitive layers. The electron-sensitive layer 5 is calibrated by a separate source of electrons (not shown in the figures) which has an electron energy which is adjusted so that it is equal to the energy of the accelerated secondary electrons. The electron-sensitive layer 5 is less apt to be mechanically damaged than is the Schumann type of photographic plate utilized for ion recording in known apparatus.

A diaphragm 7 having an elongated slit, slot, window or aperture 7' formed therethrough is positioned substantially parallel to each of the base plate 6 and layer 5 and the member 3b of the image converter electrode 3 and between and spaced from each of said layer and said image converter electrode. A secondary electro-m emitter foil 8 (shown in exaggerate thickness in FIG. 2) is positioned between and spaced from each of the electron-sen sitive layer 5 and the diaphragm 7 and is substantially parallel to said layer and to said diaphragm. The secondary electrom emitter foil 8 is penetrated by the secondary electrons released at the image converter electrode 3 before said secondary electrodes impinge upon the electron-sensitive layer 5. A distinct scaled-down image of the primary ion spectrum is produced on the electronsensitive layer 5 due to the magnetic field.

The mass spectrometer apparatus of the present invention provides increased beam current density and increased beam currents which improve the detection sen sitivity and measuring accuracy of the apparatus. The secondary electron emitter foil 8 of the mass spectrometer apparatus of the present invention further increases the secondary electron current emitted at the image converter electrode 3. Since the secondary electron emitter foil 8 separates the analyzer chamber from the chamber which houses the electron-sensitive layer 5, a pressure dilference may be maintained within specific limits between said chambers.

The intensification or increasing of the electron current by the secondary electron emitter foil 8 and the recording of the secondary electrons on the electron-sensitive layer 5 at a greater sensitivity than that provided in recording ions provides a considerable decrease in analyzing time relative to known apparatus. The maintenance of a pressure difference between the analyzer chamber and the chamber housing the electron sensitive layer 5, due to the separation of said chambers by the secondary electron emitter foil 8, permits a pressure reduction in the analyzer chamber and results in a lowered spectrum background and thus in improved detection limits.

One or more components of the image converter electrode 3 is removed from the path of the ion beams 1 to form the slit, slot, window or aperture 3, for example, so that the most intense ion beams are prevented from impinging upon said image converter electrode and background blackening in the vicinity of mass lines of greatest intensity is eliminated or considerably reduced. This also permits the direct measurement of the ion current of specific ion beams during each single exposure. Furthermore, the elimination of background blackening in the vicinity of mass lines of greatest intensity results in improved detection limits for many elements of more than one order of magnitude. The exposure intervals may be made longer than usual and permit improved detection limits for all other elements, as well.

A small portion or fraction of the ion beams 1 passes through the window 3' of the member 3a of the image converter electrode 3 and the ion beams which pass through said window are intercepted by one or more Faraday cages 9 of known structure and operation, or by one or more secondary electron multipliers. The ion currents of specific mass lines passed through the window 3' are directly measured by means of the Faraday cage 9 at the same time that the electron-sensitive layer 5 is exposed to the remaining mass spectrum.

The mass spectrometer apparatus of the present invention thus permits electrometric measurement of one or more defined ion beams 1 with simultaneous exposure of the electron-sensitive layer 5 to the mass spectra by the secondary electrons. This also results in a reduction of errors in measurement compared with known apparatus; that is, by direct measurement of the singly charged atomic ions of the parent substance and by increased accuracy of measurement which distinguishes the electron-sensitive layer 5 from an ion-sensitive layer.

While the invention has been described by means of a specific example and in a specific embodiment, we do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In mass spectrometer apparatus, image converter electrode means comprising an image converter electrode having a determined part positioned to deflect electrons in a determined direction;

ion source means for directing a plurality of ion beams to impinge in a primary ion spectrum upon the determined part of said image converter electrode in a manner whereby said ion beams release secondary electrons upon striking said determined part of said image converter electrode and said secondary electrons are emitted in said determined direction;

an electron-sensitive layer spaced from said image converter electrode in operative proximity with the determined part thereof;

voltage means connected between said image converter electrode and said electron-sensitive layer for providing an electric field in said determined direction for accelerating said secondary electrons; and magnet means providing a magnetic field between said image converter electrode and said electron-sensitive layer for guiding said secondary electrons to said electron-sensitive layer to produce a scaled-down image of said primary ion spectrum thereon.

2. In mass spectrometer apparatus as claimed in claim 1, wherein said image converter electrode means further comprises secondary electron emitter foil means positioned between and in spaced relation from said image converter electrode and said electron-sensitive layer for increasing the secondary electron current released at said image converter electrode.

3. In mass spectrometer apparatus as claimed in claim 2, wherein said mass spectrometer apparatus has a focussing plane and wherein the determined part of the image converter electrode of said image converter electrode means is positioned at an angle of less than 45 with said focussing plane.

4. In mass spectrometer apparatus as claimed in claim 3, wherein said ion beams are directed by said ion source means along a beam path and wherein a window is formed through the determined part of the image converter electrode of said image converter electrode means in said beam path in a position wherein selected ones of said ion beams pass through said window.

5. In mass spectrometer apparatus as claimed in claim 4, wherein said mass spectrometer apparatus further comprises Faraday cage means positioned adjacent the window of said image converter electrode for collection ion beams passing through said window and providing direct electrometric measurement thereof.

6. In mass spectrometer apparatus as claimed in claim 2, wherein said mass spectrometer apparatus comprises an analyzer chamber housing said image converter electrode means and a layer chamber adjacent said analyzer chamber housing said electron-sensitive layer and wherein the secondary electron emitter foil means of said image converter electrode means separates said analyzer and layer chambers and permits a determined pressure difiFerence to be maintained between said analyzer and layer chambers.

7. In mass spectrometer apparatus as claimed in claim 1, wherein the electron-sensitive layer of said image converter electrode means comprises: photosensitive means for recording the secondary electron spectrum derived from said primary ion spectrum.

8. In a method of mass spectrometer analysis, the steps of releasing secondary electrons in a secondary spectrum from a plurality of ion beams in a primary ion spectrum; electrostatically accelerating said secondary electrons in a determined direction; magnetically guiding said secondary electrons in said determined direction; recording said secondary electron spectrum; preventing determined ones of said ion beams from releasing secondary electrons; and the further steps of collecting said determined others of said ion beams and directly electrometrically measuring said determined others of said ion beams.

9. In a method of mass spectrometer analysis, the steps of releasing secondary electrons in a secondary spectrum from a plurality of ion beams in a primary ion spectrum; electrostatically accelerating said secondary electrons in a determined direction; magnetically guiding said secondary electrons in said determined direction; recording said secondary electron spectrum; preventing determined ones of said ion beams from releasing secondary electrons; and the further step of eliminating background blackening in the vicinity of mass lines of greatest intensity.

References Cited UNITED STATES PATENTS 2,894,160 7/ 1959 Sheldon. 3,041,453 6/ 1962 Daly. 3,217,161 11/1965 Craig. 3,277,297 10/ 1966 Forrester et a1.

RALPH G. NILSON, Primary Examiner A. L. BIRCH, Assistant Examiner 

